A transistor can be viewed as the atomic processing unit of a semiconductor chip. Billions of transistors are presently being implemented on a single semiconductor chip to perform extremely complex functions (such as multiple general purpose processing cores interconnected with a memory controller, I/O control hub, graphics processor, etc.). The faster the transistors are able to “switch” the faster the semiconductor chip's various logic functions will be able to operate.
Threshold voltage (VT) and gain (B) are two transistor parameters that are strongly correlated to the speed at which a transistor can switch. The threshold voltage is the voltage applied at the transistor's gate electrode at which the transistor will turn “on” (more specifically, carriers are generated in the transistor's channel). The gain of a transistor is the degree to which a transistor will amplify an input signal (in the case of a traditional field effect transistor, the gain is referred to as the “transconductance” which specifies how much drain/source output current will be generated for an applied input gate/source voltage).
As a transistor continues to operate over an extended operating time its threshold voltage and/or gain (typically both) will degrade. Specifically, the threshold voltage will drift upward which in turn causes a need to apply higher gate voltages to “turn on” the transistor. Additionally, the gain will drift lower which in turn causes a need to apply higher drain to source voltages and/or higher gate voltages to generate the same output current.
Thus, as a consequence of the transistor's degradation, the transistor's speed diminishes over the course of its lifetime. As a consequence of the transistor's drop in speed over its lifetime, semiconductor chips have traditionally been “rated” for a maximum speed that corresponds to the fastest that its transistors can reliably operate at the end of the expected lifetime of the chip. That is, the maximum speed rating of the semiconductor chip is based on how slow the transistors are expected to operate towards the end of their life. Because transistors can operate at much faster speeds in the beginning of their life, there is some efficiency loss by rating maximum speed on the slower, end of life transistor speed.